1. Field of the Invention
The present invention relates generally to a communication system using an Orthogonal Frequency Division Multiplexing (OFDM) scheme, and in particular, to an apparatus and method for transmitting/receiving pilot signals used for distinguishing base stations and sectors.
2. Description of the Related Art
Extensive research is being conducted in the 4th generation (4G) communication system, which is the next generation communication system, to provide users with services having various Qualities-of-Service (QoSs) at high data rates. Particularly, a study of the 4G communication system is being performed to provide a high-speed service capable of supporting the mobility and QoS in a Broadband Wireless Access (BWA) communication system such as a wireless Local Area Network (LAN) system and a wireless Metropolitan Area Network (MAN) system.
In the 4G communication system, a study of an OFDM scheme is being conducted as an appropriate scheme for high-speed data transmission in a wire/wireless channel. The OFDM scheme, a typical scheme for transmitting data using multiple carriers, is based on a Multi-Carrier Modulation (MCM) scheme for parallel-converting a serial input symbol stream and modulating each of the symbols with the multiple orthogonal subcarriers before transmission.
In order to provide a high-speed, high-quality wireless multimedia service, the 4G communication system requires broadband spectrum resources. The use of the broadband spectrum resources considerably increases a fading effect in a wireless transmission path due to multipath propagation and causes a frequency-selective fading effect in the transmission frequency band. For the high-speed wireless multimedia service, the OFDM scheme, which is robust against frequency selective fading, tends to be more popularly used in the 4G communication system, as it has a higher gain then a Code Division Multiple Access (CDMA) scheme.
The operations of a transmitter and a receiver for a communication system using the OFDM scheme (“OFDM communication system”) will now be described.
In the transmitter of the OFDM communication system, input data is modulated with subcarriers through a scrambler, an encoder and an interleaver. The transmitter provides a variable data rate, and operates at different coding rates, interleaving sizes and modulation schemes depending on the data rate. Commonly, the encoder uses a coding rate of ½ or ¾, and a size of the interleaver for preventing a burst error is determined according to the Number of Coded Bits per Symbol (NCBPS). The transmitter uses one of a Quadrature Phase Shift Keying (QPSK) scheme, an 8-ary Phase Shift Keying (8PSK) scheme, a 16-ary Quadrature Amplitude Modulation (16QAM) scheme and a 64-ary Quadrature Amplitude Modulation (64QAM) scheme as the modulation scheme according to the data rate.
A predetermined number of pilot subcarrier signals are added to the signals modulated by the above elements with a predetermined number of subcarrier signals, and generated into one OFDM symbol through an inverse fast Fourier transform (IFFT) operation in an IFFT block. A guard interval signal for removing inter-symbol interference in a multipath channel environment is inserted into the OFDM symbol, and then is finally input to a radio frequency (RF) processor through a symbol generator. The RF processor RF-processes an input signal and transmits the RF signal.
The guard interval signal is inserted to prevent inter-symbol interference between an OFDM symbol transmitted at a previous OFDM symbol time and an OFDM symbol transmitted at a current OFDM symbol time. The guard interval is inserted with one of a ‘Cyclic Prefix’ method and a ‘Cyclic Postfix’ method. The Cyclic Prefix method copies a predetermined number of last samples of a time-domain OFDM symbol and inserts the copied samples into an effective OFDM symbol, and the Cyclic Postfix method copies a predetermined number of first samples of a time-domain OFDM symbol and inserts the copied samples into an effective OFDM symbol.
In the receiver of the OFDM communication system, a reverse process for the process performed in the transmitter is performed. A synchronization process is also performed in the receiver. For a received OFDM symbol, a process of estimating a frequency offset and a symbol offset using a predetermined training symbol must be performed. A guard interval-removed data symbol is restored into the subcarrier signals to which pilot subcarrier signals are added, through a fast Fourier transform (FFT) block.
In order to overcome a path delay phenomenon in an actual radio channel, an equalizer estimates channel conditions for a received channel signal, and removes signal distortion in the actual radio channel from the received channel signal. The data channel-estimated through the equalizer is converted into a bit stream, and the bit stream is deinterleaved by a deinterleaver, and then, output as final data through a decoder and a descrambler.
In the OFDM communication system, the transmitter, or a base station (BS), transmits pilot subcarrier signals to the receiver, or a mobile station (MS). The base station simultaneously transmits data subcarrier signals together with the pilot subcarrier signals. The reason for transmitting the pilot subcarrier signals is for synchronization acquisition, channel estimation, and base station identification. The points where the pilot subcarrier signals are transmitted are predefined between the transmitter and the receiver. As a result, the pilot subcarrier signals serve as reference signals.
A description will now be made of an operation in which a mobile station identifies its base station using the pilot subcarrier signals.
A base station transmits the pilot subcarrier signals such that they can arrive up to a cell boundary with the transmission power which is relatively higher than that of the data subcarrier signals, using a specific pilot pattern, for the following reasons. Upon its entry into a cell, the mobile station has no information on its current base station to which the mobile station currently belongs. In order to detect its current base station, the mobile station should use only the pilot subcarrier signals. The base station transmits the pilot subcarrier signals in such a manner that it transmits the pilot subcarrier signals using a particular pilot pattern so that the mobile station can detect its current base station.
The pilot pattern refers to a pattern generated by the pilot subcarrier signals that a base station transmits. That is, the pilot pattern is determined depending on a slope of the pilot subcarrier signals and a start point at which transmission of the pilot subcarrier signals starts. The OFDM communication system should be designed such that base stations included in the OFDM communication system have different pilot patterns for identification purposes. The pilot pattern is generated by considering a coherence bandwidth and a coherence time. The coherence bandwidth represents the maximum bandwidth at which it can be assumed that a channel is flat (remains unchanged) in a frequency domain. The coherence time represents the maximum time for which it can be assumed that a channel is flat (remains unchanged) in a time domain. Because it can be assumed that channels are flat in the coherence bandwidth and the coherence time, sync acquisition, channel estimation and base station identification can be achieved by simply transmitting one pilot subcarrier signal over the coherence bandwidth for the coherence time.
The transmission of only one pilot subcarrier signal can maximize transmission of data subcarrier signals, which in turn contributes to the entire system performance. The maximum frequency band over which the pilot subcarrier signals are transmitted is referred to the coherence bandwidth, and the maximum time band, i.e. the maximum OFDM symbol time band, for which the pilot subcarrier signals are transmitted, is referred to the coherence time.
Although the number of base stations included in the OFDM communication system is subject to change according to the size of the OFDM communication system, as a general rule the number of the base stations increases with the size of the OFDM communication system. For identification of the base stations, the number of pilot patterns, having different slopes and different start points, should be equal to the number of the base stations. However, the OFDM communication system should take the coherence bandwidth and the coherence time into consideration when transmitting pilot subcarrier signals in a time-frequency domain, and the pilot patterns having different slopes and different start points, generated taking into consideration the coherence bandwidth and the coherence time, are limited. When the pilot patterns are generated without having any consideration of the coherence bandwidth and the coherence time, pilot subcarrier signals representing different base stations coexist in the pilot patterns. In this case, it is impossible to identify base stations using the pilot patterns.
FIG. 1 is a diagram illustrating transmission points of pilot subcarriers based on a pilot pattern in a conventional OFDM communication system in which only one pilot subchannel is used. Referring to FIG. 1, possible slopes to use for the generation of the pilot patterns and the number thereof, i.e. possible slopes to use for the transmission of the pilot subcarrier signals and the number thereof are limited according to a coherence bandwidth 100 and a coherence time 110. If it is assumed in FIG. 1 that when the coherence bandwidth 100 is 6 and the coherence time 110 is 1, the number of slopes that can be used for pilot patterns is an integer, then the possible slopes for pilot patterns in this condition include 6 slopes of s=0 (101) to s=5 (106). That is, each of the possible slopes for pilot patterns in this condition is one of the integers inclusive of 0 to 5.
The number of possible slopes for pilot patterns being 6 means that the number of base stations that can be distinguished using the pilot patterns in the OFDM communication system is 6. In FIG. 1, an oblique-lined circle 107 represents a pilot subcarrier signal spaced apart by the coherence bandwidth 100. In conclusion, the slopes for pilot patterns are limited by the coherence bandwidth 100.
Because generation of the pilot patterns used for identifying base stations included in the OFDM communication system is limited by the coherence bandwidth and the coherence time, the possible number of pilot patterns is also limited. If the number of base stations included in the OFDM communication system increases, the number of distinguishable base stations is limited due to the limitation in the possible number of the pilot patterns.